1. Field of the Invention
The present invention is directed to a vibrator apparatus and is more particularly directed to an apparatus for vibrational conveyance employing eccentric counterweights being driven independently by motors with the system being synchronized at the desired phase angle by a time delay relay circuit.
2. Prior Art Statement
In the early development of apparatus for vibrational conveyance, such as vibrating screens and oscillating conveyors, what was known as brute-force type drives were employed. These were drives which did not operate at or near the natural frequency of the vibrating system of which the apparatus functional parts, e.g., screen, formed a part. The brute-force drives employed a circular motion-providing mechanism to achieve directional movement and required rugged, heavy duty construction of the apparatus as well as the use of high power motors.
Subsequently, vibrating apparatus were imparted motion by the use of rotating eccentric counterweights operating at or near the natural frequency of the apparatus. During startup of such mechanisms, out of phase rotation would initially occur due to the free hanging, out of place, prestart position of the counterweights. While this created an irregular and rough initial vibration, often not readily perceptible, it was observed that the counterweights "fell into synchronization" and, after initial operation, ran smoothly. This is illustrated, for example, in U.S. Pat. Nos. 2,144,382; 3,468,418 (col. 4, line 67 et seq.); 3,053,379 (e.g., col. 2 line 49 et seq., col. 3, line 8 et seq., col. 4, line 17 et seq., col. 4, line 41 etc.).
Notwithstanding the self-adjusting aspects of eccentric counterweight rotation, it became recognized that wear and tear on the apparatus was caused, in part, by slight out-of-phase operation of the counterweights in some systems. As pointed out in U.S. Pat. No. 3,449,969, the prior art apparatus was then designed with synchronized counterweights held in a fixed relative angular relationship. Such devices had independent drive motors but had their rotating members interconnected by gears, chains and equivalent mechanical elements to positively cause the rotating means and the shafts on which they are mounted, to rotate at the same speed and in predetermined angular relationship. While these devices constituted the state of the art and remain in use today, they do require the addition of unnecessary bulk (gears, chains, etc.), increased cost, expensive maintenance and generate frictional heat in the operation, especially when members are rotated at very high rates to produce high frequency vibrations.
U.S. Pat. No. 3,449,969 describes a mechanism for overcoming the foregoing problems to some extent. The patent illustrates the use of electromagnetic forces to synchronize the rotation of eccentric weights and points out that this mechanism assures synchronization during operation. It is also indicated that no two high speed motors will rotate at exactly the same r.p.m. and have exactly the same friction characteristics, field losses, and, thus, a variable speed resistor may be used with one or both motors for manual adjustment of rotational velocity.
While the invention of U.S. Pat. No. 3,449,969 overcomes the problems of bulk weight, lubrication, maintenance and heat of friction of geared or chained counterweights, and still maintains synchronized rotation, the invention fails to overcome one of the most important causes of wear and tear on a vibrational apparatus-namely, out-of-phase initial start-up. This causes minimal problems with devices arranged with suspension having three degrees of freedom, some problems with two degrees of freedom suspension and, more problems with single degree of freedom suspension systems. The proposition that the U.S. Pat. No. 3,449,969 invention does not overcome initial out-of-phase starting problems is supported by the patent itself at column 4, line 54 et seq., indicating a slow "shift" of the magnets into synchronization. It is not until this shift is completed that "absolute" synchronization is achieved.
In summary, the prior art does not teach or render obvious a simplified, low weight, low maintenance, non-heat producing, non-mechanical mechanism for avoiding out-of-phase start-up problems. Indeed, it is not even clear from the prior art that there has been a proper identification of the problem or cause of certain wear and tear on this type of apparatus, much less the particular solution to that problem described herein.